CT, short for Computed Tomography, denotes a scanning technique to use a computer system to reconstruct a CT image of an object under detection, so as to obtain a three-dimension CT image. The scanning technique is to have rays of a single axial plane pass through the object under detection, collect transmitted rays by a computer system and reconstruct an image by a three-dimension reconstruction approach, where different parts of the object under detection have different absorptivities and transmissivities to the transmitted rays. The term “CT” used herein, if not specified otherwise, refers to an X-ray CT.
CT technique has five generations according to its development. The first four generations have a scanning part that can be implemented by a movable X-ray tube and a detector (mechanical scanning mode). The third generation and the fourth generation use a spiral scanning mode, and the scanning part used therein is comprised of a X-ray tube, a detector and a scanning stand on which the X-ray tube and the detector are mounted. In operation, the X-ray tube emits X-rays to an object to be scanned from periphery of the object by moving the scanning stand at a high speed, and a scanned CT image can be obtained after reception of the detector and processing of a computer system.
The spiral Multi-detector row CT (MDCT) that comes out in the recent years substantially belongs to the fourth generation CT, and has a scanning speed that is nearly the same as the spiral single-detector row CT. The rows of detectors, however, are increased, and thus multi-row data may be obtained by rotating the X-ray rube for one round. As for the well-developed MDCT of 64 rows, it needs 0.33 s to rotate for one round, and the temporal resolution is better than 50 ms (the temporal resolution mainly depends on the scanning period, and also on the scanning coverage and the reconstruction approach in the MDCT).
The foregoing fourth generation CT has an advantage of a high spatial resolution, but it also has a disadvantage of a low temporal resolution. The main factor that limits the temporal resolution is its scanning speed. As for the most advanced spiral MDCT, the maximal scanning speed is only 0.33 s/round, which depends on the mechanical strength limit of the scanning stand and the X-ray tube. When the CT rotates at a high speed, the line speed of the X-ray rube may be up to the first cosmic velocity. In order to ensure the stability of the structure, the rotation speed of the CT has a limit.
The fifth generation CT (UFCT) has a different scanning principle from the first four generations. An advanced electron beam technology is used to generate X-rays. The anode and cathode of the bulb tube are separate. Electron beams are emitted from the electron gun at the cathode, and are accelerated to form high-energy electron beams, which pass through a focusing and magnetic deflection coil, and project on the target surface of the anode which has a form of a 210° arc, and then X-ray beams are generated. Comparing with the conventional mechanical rotation, the scanning speed may be up to 50 ms/round.
In the medical imaging application, take the heart imaging as an example. If the mechanical scanning mode is used, one position will be scanned for 2 or 3 times within one second. If the electron beam scanning mode is used, one position will be scanned for 20 times within one second. In the industry detection application, the scanning time of performing CT on a large-scale object generally is several minutes.
In the existing CT imaging devices, there are three ways to increase the scanning speed: 1. Enhance hardware performance. For example, enhance the rotation speed of the mechanical structure, increase the number of ray sources, and others. 2. Perform equivalent scanning by means of the stability of the object to be detected. For example, the gating technique is used in the heart imaging. 3. Change the scanning mode to, for example, electron beam scanning (UFCT).
All the means may speed up the scanning speed to a certain extent, but cannot achieve ultra-fast scanning and CT imaging on an object that moves at a high speed.